The present invention relates to electrography and, more specifically, to methods and apparatus for fusing electroscopic toner images onto a supporting surface.
In electrography, latent electrostatic images formed on the surface of an electrographic recording element are rendered visible by the selective deposition of electroscopic toner particles. The developed or toned image so formed may be either permanently affixed to the recording element surface or transferred to a paper receiving sheet before being permanentized.
While many techniques are known for affixing or fusing electrographic toner images to a supporting surface, including heat fusing, pressure fusing and vapor fusing, none has proven by itself, or in combination, totally satisfactory for many electrographic applications. Generally, it has been difficult to construct an entirely satisfactory heat fuser having a short warm-up time, high efficiency and ease of control. A further problem associated with heat fusers has been their tendency to burn or scorch the support material. On the other hand, pressure fusing, whether hot or cold, has created problems with image offsetting, and vapor fusing typically employs a toxic solvent which renders it commercially unfeasible for many applications.
In addition to the fusing techniques mentioned above, radiant flash fusing has been considered. One advantage of flash fusing over other known techniques is that the energy, which is propagated in the form of electromagnetic waves, is instantaneously available and requires no intervening medium for its propagation. Moreover, such apparatus requires no long warm-up time and is capable of great speed.
While having many advantages, the flash fusing technique has not been commercially adapted, primarily due to its inefficient use of available radiant energy. Typically, the toner image constitutes a relatively small percentage of the total surface area subjected to radiant energy. Because of the properties of most copy material and the geometry of flash fusing apparatus heretofore considered, much of the radiant energy available for fusing is wasted by being transmitted through the copy material or by being reflected away from the fusing area.
In addition to the inefficient use of available energy by conventional radiant flash fusing devices, another disadvantage associated therewith has been the nonuniformity of image fixing produced thereby. This problem is primarily due to the fact that it has been difficult to produce highly uniform irradiance over a large surface area from a relatively small source such as a flashlamp. Uniform irradiance over a small portion of the copy material to be fused can be achieved by positioning the flashlamp as close as possible to such copy material but this arrangement suffers the disadvantage of requiring incremental fusing of the copy sheets.
While considerable effort has been expended in providing schemes for enhancing the efficiency and uniformity of fix of electrographic flash fusing systems, most efforts have been directed toward the provision of specially contoured reflecting surfaces which are designed to at least partially surround the flashlamp and thereby conserve energy via multiple reflections. In addition to being costly to fabricate, such reflecting surfaces tend to become contaminated by loose toner particles and thereby necessitate frequent cleaning operations. The contamination becomes particularly acute when the flashlamp and reflector are placed in close proximity to the toner bearing copy material so as to provide the desired uniform irradiance thereof.